As shown in FIG. 1, in a thermoelectric power plant that generates electricity using coal, electricity is generally generated by pulverizing coal to a specific size in order to promote the burning of coal, that is, preparing pulverized coal, and feeding the pulverized coal into a boiler in which the coal is burned, and at the same time, producing high-temperature and high-pressure steam and feeding the produced steam into a steam turbine.
In particular, coal generally contains about 2-15% non-combustible ash, that is, coal ash. Thus, when pulverized ash is fed into a boiler, coal ash accounting for about 20% of the pulverized coal is melted by the high-temperature heat of combustion, a number of particles are condensed and drop to the bottom of the boiler, about 80% of the pulverized coal is burned and scatters together with the flow of flue gas, and coal ash contained in the scattered pulverized coal is collected by dust collecting unit such as an electrostatic precipitator.
The type of coal ash that drops to the bottom of the boiler is called “bottom ash”. The bottom ash particles drop to the bottom of the boiler by the weight thereof after being attached to the furnace wall, a super-heater, a reheater or the like and have a particle size of about 1-5 mm. The bottom ash particles are collected at the bottom of the boiler and ground with a grinder, and then sent to an ash transfer tank. The bottom ash particles in the ash transfer tanks are mostly disposed of in an ash landfill. Because the bottom ash has non-uniform particle size and distribution and inferior quality, it is completely is landfilled in breakwaters built into the sea.
Also, the type of coal ash, which scatters from the fluidized-bed boiler and is collected by the electrostatic precipitator, is called “fly ash”, which is collected in a hopper below an economizer or an air preheater and which is captured by the electrostatic precipitator and collected in a hopper below the electrostatic precipitator. The type of fly ash that is collected in the hopper below the economizer or the air preheater has a particle size of 0.3-0.1 μm and accounts for about 5% of the coal ash generated, and the type of fly ash, which is captured by the electrostatic precipitator and collected in the hopper below the electrostatic precipitator has various particle sizes depending on the type of coal or combustion conditions, accounts for about 75-80% of the coal ash generated, and is mostly recycled. Fly ash which is not recycled is sent to the ash transfer tank and disposed of in an ash landfill, like the case of the bottom ash.
The coal ash consisting of a heterogeneous mixture of fly ash and bottom ash, which are disposed of in a landfill, is called “reclaimed ash”, and the physical properties of the reclaimed ash greatly differ depending on the place and time when the reclaimed ash is landfilled. Because of this disadvantage, in order for the reclaimed ash to be used in concrete aggregates and the like, the reclaimed ash stored in an ash pond is screened according to particle size and subjected to a washing process and the like. Even if the reclaimed ash is recycled in this manner, there is a problem in that coal ash remaining after the screening process should be disposed of.
Also, the use of only a portion of reclaimed ash from an additional process for recycling the reclaimed ash causes an increase in the production cost of aggregates and is undesirable in terms of the efficiency of resource recycling.
In Korea, about 1,100 thousand tons of coal ash was generated in the year 2002, and the reclamation of coal ash has problems in that it is difficult to ensure and maintain a landfill, residents neighboring the landfill suffer inconvenience due to fugitive dust, and the reclaimed coal ash flows out in the rainy season to contaminate the surrounding environment.
U.S. Pat. No. 5,992,336 discloses reburning a mixture of bottom ash and coal to reduce fuel expenses and waste. However, there is a problem in that the coal ash generated should be disposed of after the reburning process.
Also, Korean Patent Registration No. 0447739 discloses a concrete composition comprising bottom ash and a preparation method thereof, in which bottom ash is separated into coarse aggregate and fine aggregate, which are used as concrete aggregate substitutes. Moreover, Korean Patent Registration No. 0470676 discloses a concrete composition using of bottom as as replacement of aggregate for concrete mixing, and Korean Patent Registration No. 0668953 discloses a method for correcting the accuracy, compressive strength and slump of concrete using reclaimed coal ash.
However, when reclaimed coal ash or bottom ash is recycled as a concrete aggregate substitute as described above, the reclaimed coal ash and the bottom ash have various water contents depending on particle size when used as a concrete aggregate and makes it difficult to ensure the slump and strength of concrete manufactured therefrom, because they contain unrefined fly ash at various ratios and have a very non-uniform particle size distribution, and the strength of the bottom ash differs depending on particle size and shape. Thus, it is significantly difficult to practically use reclaimed coal ash in concrete. Also, in order to manufacture concrete from reclaimed ash or bottom ash in a stable manner, it is necessary to carry out a process of separating the reclaimed ash or the bottom ash into coarse aggregate and fine aggregate after a washing process, but these additional processes such as the particle size separation process can increase the production cost of the aggregate and are undesirable in terms of the efficiency of resource recycling. For these reasons, the use of reclaimed coal ash has not provided satisfactory results in economic and technical terms.
Accordingly, the present inventors have made many efforts to solve the above-described problems occurring in the prior art and, as a result, have found that, when a coal ash recycling apparatus comprising a unburned coal ash recovery unit and a lightweight aggregate manufacturing unit is provided in a thermoelectric power plant and is used to recycle various types of coal ash (fly ash, bottom ash, reclaimed ash, etc.) which are generated in the thermoelectric power plant, each type of coal ash which is generated in the thermoelectric power plant can be completely recycled for specific uses, unlike the prior art in which each type of coal ash is partially recycled, thereby completing the present invention.